JP2006245398A - Electret structure and its forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electret structure having an excellent charge holding capacity. <P>SOLUTION: A silicon oxide film 1 as an electret is formed on an insulating film 3 in a patterning. The insulating film 2 is formed so as to coat the silicon oxide film 1. The upper end of the silicon oxide film 1 is formed in a round shape while a section coating the upper end of the silicon oxide film 1 in the insulating film 2 is also formed in the round shape. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electret structure in an electret condenser using a vibrating membrane.

  Conventionally, as an electret element, which is a dielectric material having permanent electric polarization, applied to an element such as a condenser microphone, an organic polymer such as FEP (fluorinated ethylene propylene copolymer) or polytetrafluoroethylene is used. Was used. However, since electret elements using these materials have poor heat resistance, there is a problem that they are difficult to use as reflow elements mounted on a substrate.

  In recent years, in order to achieve thinning or miniaturization of electrets using microfabrication technology, electrets using silicon oxide films as shown in Patent Document 1 have been proposed instead of organic polymer polymers. ing.

Specifically, in Patent Document 1, after a silicon oxide film is formed on the surface of a substrate, the silicon oxide film is not released from the film formation atmosphere to the atmosphere, and in a gas atmosphere not containing moisture and containing oxygen. In contrast, it is disclosed that an electret is formed by performing a heat treatment at 200 ° C. to 400 ° C. and then performing a charging process on the silicon oxide film.
JP 2002-33241 A

  However, electrets have the disadvantage that charges are released when they come into contact with the liquid. For example, when electretized FEP is immersed in ethanol, the charge accumulated in the FEP does not become zero, but significantly decreases. According to the experiments by the inventors of the present application, when an FEP having a surface potential indicating a charge amount of 300 V (specifically, an FEP formed with a thickness of 12.5 μm on a stainless steel substrate) is immersed in ethanol, the surface potential is several. It decreased to about V. This phenomenon occurs similarly when the electret is immersed not only in ethanol but also in another organic solvent or water. In addition, the material is not a phenomenon peculiar to FEP but a phenomenon that occurs in general electret materials such as a silicon oxide film.

  In view of the above, an object of the present invention is to provide an electret structure with excellent charge retention capability. Another object of the present invention is to provide a small ECM (electret condenser microphone) that does not require a charge supply circuit by manufacturing an electret condenser composed of electrets having a permanent charge by MEMS (Micro Electro Mechanical Systems) technology. .

  In order to achieve the above object, a first electret structure according to the present invention includes a first insulating film, a silicon oxide film patterned on the first insulating film and storing charges, And a second insulating film formed so as to cover the silicon oxide film, and the upper end portion of the silicon oxide film is rounded.

  The second electret structure according to the present invention is formed so as to cover the first insulating film, the silicon oxide film patterned and stored on the first insulating film, and the silicon oxide film. And a portion covering the upper end of the silicon oxide film in the second insulating film is rounded.

  In the second electret structure of the present invention, the upper end portion of the silicon oxide film is preferably rounded.

  1st or 2nd electret structure of this invention WHEREIN: The film thickness of the part which covers the side surface of the said silicon oxide film in the said 2nd insulating film is a part which covers the upper surface of the said silicon oxide film in the said 2nd insulating film It is preferable that it is larger than the film thickness.

  In the first or second electret structure of the present invention, the first insulating film and the second insulating film are preferably silicon nitride films formed by a low pressure CVD method.

  The first electret structure forming method according to the present invention includes a step (a) of patterning and forming a first silicon oxide film on the first insulating film, and a second so as to cover the first silicon oxide film. The step (b) of forming the silicon oxide film and dry etching is performed on the entire surface of the second silicon oxide film, so that the second silicon oxide film is formed on the side surface of the first silicon oxide film. A step (c) of forming a side wall, and a step (d) of forming a second insulating film so as to cover the first silicon oxide film and the side wall.

  The second electret structure forming method according to the present invention includes a step (a) of forming a silicon oxide film on the first insulating film, and a step of forming a mask pattern covering a predetermined region in the silicon oxide film ( b), a step (c) of selectively removing the upper portion of the silicon oxide film by wet etching using the mask pattern after the step (b), and a step of the mask after the step (c). A step (d) of patterning the silicon oxide film by dry etching using a pattern, and a second insulating film is formed after the step (d) so as to cover the patterned silicon oxide film Step (e).

  In the first or second electret structure forming method of the present invention, it is preferable that the first insulating film and the second insulating film are silicon nitride films.

  The third electret structure forming method according to the present invention includes a step (a) of patterning a silicon oxide film on the first insulating film, and a second insulating film so as to cover the silicon oxide film. Performing the step (b), the step (c) of forming a third insulating film so as to cover the second insulating film, and performing dry etching on the entire surface of the third insulating film; And (d) forming a sidewall made of the third insulating film on the side surface of the second insulating film.

  In the third electret structure forming method of the present invention, the first insulating film, the second insulating film, and the third insulating film are preferably silicon nitride films.

  According to the present invention, since the silicon oxide film serving as the electret is covered with the insulating film, the moisture resistance of the electret can be increased, thereby improving the charge retention capability of the electret. In addition, since the shape of the upper end portion of the silicon oxide film or the insulating film covering the silicon oxide film is rounded, discharge due to electric field concentration at the upper end portion can be reduced in the electretization process, and step coverage and film quality of the insulating film can be reduced. Stability can be improved. Therefore, since the moisture resistance of the electret and thus the charge retention capability can be further improved, a highly reliable and compact electret condenser can be realized and various applications including a microphone equipped with the electret condenser can be realized. The device can be widely supplied to society. In particular, it is possible to improve the temporal reliability of ECM using the electret condenser.

  Furthermore, by manufacturing an electret condenser composed of the electret of the present invention by the MEMS technology, a small-sized ECM that does not require a charge supply circuit can be provided.

  First, an electret condenser microphone (ECM) having an electret structure according to each embodiment of the present invention described later will be described with reference to the drawings.

  1A and 1B are configuration diagrams of an ECM having an electret structure according to each embodiment of the present invention, FIG. 1A is a plan view of the ECM, and FIG. It is sectional drawing of ECM.

  As shown in FIGS. 1A and 1B, the ECM has a microphone portion 17, an SMD (surface mount component) 18 such as a capacitor, and an FET (field effect transistor) portion 19 mounted on a printed circuit board 20. Is made up of. Although not shown in FIG. 1A, as shown in FIG. 1B, the printed circuit board 20 on which the microphone unit 17, the SMD 18 and the FET unit 19 are mounted is protected by a case 21. Yes.

  FIG. 2 is a circuit block diagram of the ECM shown in FIGS.

  As shown in FIG. 2, the internal circuit 22 of the ECM is composed of a microphone unit 17, an SMD 18, and an FET unit 19 made of electret capacitors. In addition, signals are output from the output terminal 23 and the output terminal 24 of the internal circuit 22 to the external terminal 25 and the external terminal 26. In actual operation, when a signal having a voltage of about 2 V, for example, is input from the terminal 27 connected to the external terminal 25 via a resistor, for example, several terminals are connected to the terminal 28 connected to the external terminal 25 via a capacitor. A signal having an AC voltage of 10 mV is output. Each of the external terminal 26 and the terminal 29 connected thereto is connected to an output terminal 24 that is a GND terminal in the ECM internal circuit 22.

  Next, the operation principle of ECM will be described with reference to FIG. 3 and FIGS. Here, FIG. 3 is a cross-sectional view of an electret condenser to be the microphone section 17, that is, an electret condenser using an electret structure of each embodiment of the present invention described later.

  As shown in FIG. 3, a conductive film 10 is formed on a semiconductor substrate 4 having a cavity so as to cover the cavity. The conductive film 10 is electrically connected to the gate of the FET 19 shown in FIG. On the conductive film 10, a silicon oxide film 1 having an upper surface and side surfaces covered with an insulating film 2 and a lower surface covered with an insulating film 3 is provided. The silicon oxide film 1 becomes an electret that stores electric charges. As the insulating films 2 and 3, for example, a silicon nitride film having etching resistance and moisture resistance with respect to an HF-based solution is used. The conductive film 10, the silicon oxide film 1, and the insulating films 2 and 3 constitute a vibration film 11.

  A conductive film 8 supported by the insulating film 5 is provided above the semiconductor substrate 4 so that an air gap is interposed between the vibration film 11 and the semiconductor substrate 4. The conductive film 8 becomes a fixed film and has a sound hole 9. As the conductive film 8, for example, a low-resistance polysilicon film having etching resistance against HF-based solutions is used. The conductive film 8 is electrically connected to a terminal 29 which is a GND terminal shown in FIG.

In the electret condenser shown in FIG. 3, when sound pressure is received from above the semiconductor substrate 4, that is, when sound pressure is received through the sound hole 9 of the conductive film 8, the vibration film 11 is mechanically driven according to the magnitude of the sound pressure. Vibrate. The electret capacitor shown in FIG. 3 has a parallel plate type capacitor structure in which the conductive film 8 and the conductive film 10 are electrodes, respectively. When the vibration film 11 vibrates, the distance between the electrodes changes. The capacity (C) changes. Here, since the amount of charge (Q) stored in the capacitor is constant, when the capacitance (C) changes, the voltage (V) between the conductive film 8 and the conductive film 10 changes. The reason for this is physically below (Formula 1)
Q = C · V (Formula 1)
This is because the condition expressed by

  Since the conductive film 10 is electrically connected to the gate of the FET 19, the gate potential of the FET 19 also changes when the conductive film 10, that is, the vibration film 11 vibrates. Then, a change in the potential of the gate of the FET 19 is output to the terminal 28 as a change in voltage.

  The capacitance of the electret capacitor shown in FIG. 3, that is, the capacitance of the capacitor structure in the ECM shown in FIGS. 1A and 1B and FIG. It is determined by the capacitance component that does not change. Therefore, when the capacitance component that does not change changes depending on the external environment, the performance of the ECM is greatly deteriorated.

  In particular, since the silicon oxide film 1 used in the electret structure of each embodiment of the present invention to be described later is a material that absorbs moisture in the atmosphere, etc., the silicon oxide film 1 that has been electretized is in the atmosphere. The exposure to the temperature lowers the reliability of the ECM over time.

  On the other hand, according to the electret structure of each embodiment of the present invention described below, the charge retention characteristics of the electretized silicon oxide film 1 can be improved, so that the time-dependent reliability of ECM is improved. Is possible. Further, by manufacturing an electret capacitor composed of electrets having a permanent charge using the electret structure by the MEMS technology, it is possible to realize a small and high-performance ECM that does not require a charge supply circuit.

  In the electret capacitor shown in FIG. 3, the silicon oxide film 1 serving as the electret is provided on the vibration film 11 side. Instead, the silicon oxide film 1 is provided on the fixed film (conductive film 8) side. Also in that case, it goes without saying that the electret structure of each embodiment of the present invention to be described later can be adopted.

(First embodiment)
Hereinafter, an electret structure according to a first embodiment of the present invention will be described with reference to the drawings.

  FIG. 4 is a cross-sectional view of the electret structure according to the first embodiment. In FIG. 4, constituent elements corresponding to those of the electret condenser shown in FIG. 3 are denoted by the same reference numerals.

  As shown in FIG. 4, a silicon oxide film 1 that serves as an electret (stores charges) is formed on the insulating film 3 by patterning. An insulating film 2 is formed so as to cover the upper surface and side surfaces of the silicon oxide film 1. As the insulating film 2, for example, a silicon nitride film having a dense film quality is used. Similarly, as the insulating film 3 covering the lower surface of the silicon oxide film 1, for example, a silicon nitride film is used. The silicon oxide film 1 is completely covered with insulating films 2 and 3.

  The feature of this embodiment is that the end portion (hereinafter referred to as the upper end portion) of the upper surface of the silicon oxide film 1 is formed in a round shape, and the portion covering the upper end portion of the silicon oxide film 1 in the insulating film 2 is also formed in a round shape. It has been done.

  According to the first embodiment, since the silicon oxide film 1 serving as the electret is covered with the insulating films 2 and 3, the moisture resistance of the electret can be increased, thereby improving the charge retention capability of the electret. Further, since the shapes of the upper end portions of the silicon oxide film 1 and the insulating film 2 covering the silicon oxide film 1 are rounded, in other words, the silicon oxide film 1 and the insulating film 2 are each formed into a shape having no sharp corners. Therefore, the discharge caused by the electric field concentration at the upper end can be mitigated and the step coverage and film quality stability of the insulating film 2 can be improved. Therefore, since the moisture resistance of the electret and thus the charge retention capability can be further improved, it is possible to realize a highly reliable small and high-performance electret condenser and various applications including a microphone equipped with the electret condenser. The device can be widely supplied to society. In particular, it is possible to improve the temporal reliability of ECM using the electret condenser.

  Furthermore, by manufacturing the electret capacitor using the electret structure of this embodiment by the MEMS technology, a charge supply circuit is unnecessary and a small ECM can be provided.

  In the present embodiment, it is preferable to use, for example, a silicon nitride film formed by a low pressure CVD (chemical vapor deposition) method as the insulating films 2 and 3. If it does in this way, the moisture resistance of an electret can be made high reliably.

  In the present embodiment, when a through hole such as a sound hole or a leak hole is formed in the silicon oxide film 1 covered with the insulating films 2 and 3, the vertical plane shape and horizontal plane shape of the through hole are also as follows. It is desirable to have a round shape with no sharp corners. If it does in this way, the discharge resulting from the electric field concentration in the upper end part of a through-hole can be relieved by the electretization process.

  FIGS. 5A and 5B are diagrams respectively showing a vertical plane shape and a horizontal plane shape of the electret structure when through holes are formed in the electret structure shown in FIG. 4. Specifically, FIG. 5A corresponds to the cross-sectional view of the electret structure shown in FIG. 4, and shows a vertical surface shape of the electret structure including the through holes. FIG. 5B is a cross-sectional view taken along line A-A ′ in FIG. 5A, and shows a horizontal plane shape of an electret structure including a through hole. FIG. 5A is also a cross-sectional view taken along line B-B ′ in FIG. As shown in FIGS. 5A and 5B, a through hole 12 is formed in the silicon oxide film 1 covered with the insulating films 2 and 3. Here, the silicon oxide film 1 is completely covered with the insulating films 2 and 3, including the portion that becomes the wall surface of the through hole 12. Further, the upper end portion of the silicon oxide film 1 and each corner portion 36 of the through-hole forming region are rounded both in the vertical plane shape and in the horizontal plane shape. That is, the through-hole 12 has a round vertical surface shape and a horizontal surface shape without sharp corners. The electret structure as shown in FIGS. 5A and 5B can further improve the charge retention capability of the electret.

(Second Embodiment)
Hereinafter, an electret structure forming method according to the second embodiment of the present invention will be described with reference to the drawings.

  FIGS. 6A to 6E are cross-sectional views showing respective steps of the electret structure forming method according to the second embodiment. In addition, the formation method of the electret structure which concerns on 2nd Embodiment is an example of the method for forming the electret structure which concerns on 1st Embodiment. 6A to 6E, components corresponding to those of the electret condenser shown in FIG. 3 are denoted by the same reference numerals.

  First, as shown in FIG. 6A, an insulating film 3 made of a silicon nitride film is formed on a semiconductor substrate 4 by, eg, CVD. Subsequently, as shown in FIG. 6B, after the silicon oxide film 1 is formed on the insulating film 3 by, for example, the CVD method, the silicon oxide film 1 is patterned into a predetermined shape.

  Thereafter, as shown in FIG. 6C, a silicon oxide film 31 is formed so as to cover the patterned silicon oxide film 1. Subsequently, by performing dry etching on the entire surface of the silicon oxide film 31 on the semiconductor substrate 4, as shown in FIG. 6D, a side wall 31A made of the silicon oxide film 31 is formed on the side surface of the silicon oxide film 1. Form. As a result, the upper end portion of the silicon oxide film 1 including the sidewall 31A can be rounded.

  Next, as shown in FIG. 6E, an insulating film 2 made of a silicon nitride film is formed by, for example, a CVD method so as to completely cover the silicon oxide film 1 and the sidewall 31A. Thereafter, the silicon oxide film 1 is charged by, for example, plasma discharge or corona discharge to form an electret.

  According to the second embodiment, since the upper end portion of the silicon oxide film 1 has a round shape without sharp corners, the discharge caused by the electric field concentration at the upper end portion can be mitigated and the silicon oxide film 1 can be insulated. The step coverage and film quality stability of the film 2 can be improved. Accordingly, an electret having excellent moisture resistance and excellent charge retention characteristics can be formed.

  In the present embodiment, as a method for forming the insulating films 2 and 3, for example, a low pressure CVD method is preferably used. If it does in this way, the moisture resistance of an electret can be made high reliably.

(Third embodiment)
Hereinafter, an electret structure forming method according to a third embodiment of the present invention will be described with reference to the drawings.

  7A to 7D are cross-sectional views showing respective steps of the electret structure forming method according to the third embodiment. In addition, the formation method of the electret structure which concerns on 3rd Embodiment is another example of the method for forming the electret structure which concerns on 1st Embodiment. Further, in FIGS. 7A to 7D, the same reference numerals are given to the components corresponding to the components of the electret condenser shown in FIG.

  First, as shown in FIG. 7A, an insulating film 3 made of a silicon nitride film is formed on a semiconductor substrate 4 by, for example, a CVD method. Subsequently, after the silicon oxide film 1 is formed on the insulating film 3 by, for example, the CVD method, a resist film 34 covering a predetermined region in the silicon oxide film 1 is formed.

  Thereafter, as shown in FIG. 7B, the upper portion of the silicon oxide film 1 is selectively removed by wet etching using the resist film 34 as a mask. Subsequently, as shown in FIG. 7C, after the silicon oxide film 1 is patterned by dry etching using the resist film 34 as a mask, the resist film 34 is removed. Thereby, the sharp corner of the upper end of the silicon oxide film 1 can be removed.

  Next, as shown in FIG. 7D, an insulating film 2 made of a silicon nitride film is formed by, eg, CVD so as to completely cover the silicon oxide film 1. Thereafter, the silicon oxide film 1 is charged by, for example, plasma discharge or corona discharge to form an electret.

  According to the third embodiment, since the sharp corner of the upper end portion of the silicon oxide film 1 is removed, the discharge due to the electric field concentration at the upper end portion can be reduced in the electret process and the silicon oxide film 1 is covered. The step coverage and film quality stability of the insulating film 2 can be improved. Accordingly, an electret having excellent moisture resistance and excellent charge retention characteristics can be formed.

  In the present embodiment, as a method for forming the insulating films 2 and 3, for example, a low pressure CVD method is preferably used. If it does in this way, the moisture resistance of an electret can be made high reliably.

(Fourth embodiment)
Hereinafter, an electret structure according to a fourth embodiment of the present invention will be described with reference to the drawings.

  FIG. 8 is a cross-sectional view of an electret structure according to the fourth embodiment. In FIG. 8, components corresponding to those of the electret condenser shown in FIG. 3 are denoted by the same reference numerals.

  As shown in FIG. 8, a silicon oxide film 1 that serves as an electret (stores charges) is formed on the insulating film 3 by patterning. An insulating film 2 is formed so as to cover the upper surface and side surfaces of the silicon oxide film 1. As the insulating film 2, for example, a silicon nitride film having a dense film quality is used. Similarly, as the insulating film 3 covering the lower surface of the silicon oxide film 1, for example, a silicon nitride film is used. The silicon oxide film 1 is completely covered with insulating films 2 and 3.

  A feature of this embodiment is that a sidewall 35A made of, for example, a silicon nitride film is formed on the side surface of the insulating film 2. Note that the region near the upper end portion of the silicon oxide film 1 in the insulating film 2 including the side wall 35A has a round shape without sharp corners by etching for forming the side wall 35A. Further, the film thickness of the portion covering the side surface of the silicon oxide film 1 in the insulating film 2 including the side wall 35A is larger than the film thickness of the portion covering the upper surface of the silicon oxide film 1 in the insulating film 2.

  According to the fourth embodiment, since the silicon oxide film 1 serving as an electret is covered with the insulating films 2 and 3, the electret can be improved in moisture resistance, and thereby the charge retention capability of the electret can be improved. Further, since the shape of the upper end portion of the insulating film 2 (including the side wall 35A) covering the silicon oxide film 1 is rounded, in other words, the insulating film 2 has a shape without sharp corners. Discharge due to electric field concentration at the upper end can be mitigated, and step coverage and film quality stability of the insulating film 2 can be improved. Therefore, since the moisture resistance of the electret and thus the charge retention capability can be further improved, a highly reliable and compact electret condenser can be realized and various applications including a microphone equipped with the electret condenser can be realized. The device can be widely supplied to society. In particular, it is possible to improve the temporal reliability of ECM using the electret condenser.

  Furthermore, by manufacturing the electret capacitor using the electret structure of this embodiment by the MEMS technology, a charge supply circuit is unnecessary and a small ECM can be provided.

  In the present embodiment, it is preferable to use, for example, a silicon nitride film formed by a low pressure CVD method as the insulating films 2 and 3. If it does in this way, the moisture resistance of an electret can be made high reliably.

  In the present embodiment, when a through hole such as a sound hole or a leak hole is formed in the silicon oxide film 1 covered with the insulating films 2 and 3, the vertical plane shape and horizontal plane shape of the through hole are also as follows. It is desirable to have a round shape with no sharp corners. If it does in this way, the discharge resulting from the electric field concentration in the upper end part of a through-hole can be relieved by the electretization process.

  5 (a) and 5 (b) are diagrams respectively showing a vertical surface shape and a horizontal surface shape of the electret structure when a through hole is formed in the electret structure shown in FIG. 8 (however, illustration of the side wall 35A is omitted). is doing). Specifically, FIG. 5A corresponds to the cross-sectional view of the electret structure shown in FIG. 8, and shows a vertical surface shape of the electret structure including the through holes. FIG. 5B is a cross-sectional view taken along line A-A ′ in FIG. 5A, and shows a horizontal plane shape of an electret structure including a through hole. FIG. 5A is also a cross-sectional view taken along line B-B ′ in FIG. As shown in FIGS. 5A and 5B, a through hole 12 is formed in the silicon oxide film 1 covered with the insulating films 2 and 3. Here, the silicon oxide film 1 is completely covered with the insulating films 2 and 3, including the portion that becomes the wall surface of the through hole 12. Further, the upper end portion of the silicon oxide film 1 and each corner portion 36 of the through-hole forming region are rounded both in the vertical plane shape and in the horizontal plane shape. That is, the through-hole 12 has a round vertical surface shape and a horizontal surface shape without sharp corners. The electret structure as shown in FIGS. 5A and 5B can further improve the charge retention capability of the electret.

(Fifth embodiment)
Hereinafter, an electret structure forming method according to a fifth embodiment of the present invention will be described with reference to the drawings.

  9A to 9D are cross-sectional views showing respective steps of the electret structure forming method according to the fifth embodiment. In addition, the formation method of the electret structure which concerns on 5th Embodiment is an example of the method for forming the electret structure which concerns on 4th Embodiment. Further, in FIGS. 9A to 9D, components corresponding to the components of the electret condenser shown in FIG.

  First, as shown in FIG. 9A, an insulating film 3 made of a silicon nitride film is formed on a semiconductor substrate 4 by, for example, a CVD method. Subsequently, after the silicon oxide film 1 is formed on the insulating film 3 by, for example, the CVD method, the silicon oxide film 1 is patterned into a predetermined shape.

  Thereafter, as shown in FIG. 9B, an insulating film 2 made of a silicon nitride film is formed by, eg, CVD so as to cover the patterned silicon oxide film 1. Subsequently, as shown in FIG. 9C, an insulating film 35 made of a silicon nitride film is formed on the insulating film 2 by, eg, CVD.

  Next, by performing dry etching on the entire surface of the insulating film 35, sidewalls 35A made of the insulating film 35 are formed on the side surfaces of the insulating film 2 as shown in FIG. Thereby, the region near the upper end portion of the silicon oxide film 1 in the insulating film 2 including the sidewall 35A has a round shape without sharp corners. Further, the film thickness of the portion covering the side surface of the silicon oxide film 1 in the insulating film 2 including the side wall 35A is larger than the film thickness of the portion covering the upper surface of the silicon oxide film 1 in the insulating film 2.

  Thereafter, the silicon oxide film 1 is charged by, for example, plasma discharge or corona discharge to form an electret.

  According to the fifth embodiment, the upper end portion of the insulating film 2 (including the side wall 35A) covering the silicon oxide film 1 has a round shape without sharp corners. Discharge due to electric field concentration can be mitigated, and step coverage and film quality stability of the insulating film 2 can be improved. Accordingly, an electret having excellent moisture resistance and excellent charge retention characteristics can be formed.

  In the present embodiment, as a method for forming the insulating films 2 and 3, for example, a low pressure CVD method is preferably used. If it does in this way, the moisture resistance of an electret can be made high reliably.

  By the way, since the silicon oxide film 1 used as an electret in each embodiment of the present invention described above is a material that has a remarkable adsorption of moisture in the atmosphere, the silicon oxide film 1 that has been electretized is an atmosphere. Exposure inside reduces the reliability over time of the ECM.

  On the other hand, according to each embodiment of the present invention, the silicon oxide film 1 is covered with the insulating films 2 and 3 made of, for example, a moisture-resistant silicon nitride film, thereby improving the moisture resistance of the electret and hence the charge retention capability of the electret. Can be improved.

  Further, by rounding the shape of the upper end portion of the silicon oxide film 1 or the insulating film 2 covering it, it is possible to prevent discharge due to electric field concentration at the upper end portion and improve the step coverage and film quality stability of the insulating film 2. Can be made. Therefore, it is possible to improve the reliability over time of the ECM.

  Further, by manufacturing an electret capacitor using the silicon oxide film 1 of each embodiment of the present invention as an electret having a permanent charge by MEMS technology, a small and high performance ECM can be realized without a charge supply circuit. It becomes possible.

  The present invention relates to an electret structure and a method of forming the same, and when applied to an electret in an electret condenser using a vibrating membrane, the moisture resistance and charge retention capability of the electret can be improved, and the high performance with excellent reliability. It is very useful for realizing a small ECM.

Fig.1 (a) is a top view of ECM which has the electret structure of each embodiment of this invention, FIG.1 (b) is sectional drawing of the said ECM. FIG. 2 is a circuit block diagram of the ECM shown in FIGS. FIG. 3 is a cross-sectional view of an electret condenser using the electret structure of each embodiment of the present invention. FIG. 4 is a cross-sectional view of the electret structure according to the first embodiment of the present invention. FIGS. 5A and 5B are diagrams respectively showing a vertical surface shape and a horizontal surface shape of the electret structure when through holes are formed in the electret structure according to the first and fourth embodiments of the present invention. FIGS. 6A to 6E are cross-sectional views showing respective steps of the electret structure forming method according to the second embodiment of the present invention. 7A to 7D are cross-sectional views showing respective steps of the electret structure forming method according to the third embodiment of the present invention. FIG. 8 is a cross-sectional view of an electret structure according to the fourth embodiment of the present invention. FIGS. 9A to 9D are cross-sectional views showing respective steps of the electret structure forming method according to the fifth embodiment of the present invention.

Explanation of symbols

1 Silicon oxide film 2 Insulating film (silicon nitride film)
3 Insulating film (silicon nitride film)
4 Semiconductor substrate 5 Insulating film 8 Conductive film 9 Sound hole 10 Conductive film 11 Vibration film 12 Through-hole 17 Microphone part 18 SMD
19 FET
20 Printed Circuit Board 21 Case 22 Internal Circuit 23 Output Terminal 24 Output Terminal 25 External Terminal 26 External Terminal 27 Terminal 28 Terminal 29 Terminal 31 Second Silicon Oxide Film 31A Side Wall 34 Resist Film 35 Insulating Film (Silicon Nitride Film)
35A side wall 36 corner of silicon oxide film 1

Claims (10)

A first insulating film;
A silicon oxide film patterned on the first insulating film and storing charges;
A second insulating film formed to cover the silicon oxide film,
The electret structure is characterized in that the upper end portion of the silicon oxide film is rounded. A first insulating film;
A silicon oxide film patterned on the first insulating film and storing charges;
A second insulating film formed to cover the silicon oxide film,
A portion of the second insulating film covering the upper end portion of the silicon oxide film is formed in a round shape. In the electret structure according to claim 2,
The electret structure characterized in that the upper end portion of the silicon oxide film is rounded. In the electret structure according to claim 1 or 2,
An electret characterized in that a film thickness of a portion covering the side surface of the silicon oxide film in the second insulating film is larger than a film thickness of a portion covering the upper surface of the silicon oxide film in the second insulating film. Construction. In the electret structure according to claim 1 or 2,
The electret structure, wherein the first insulating film and the second insulating film are silicon nitride films formed by a low pressure CVD method. A step (a) of patterning a first silicon oxide film on the first insulating film;
Forming a second silicon oxide film so as to cover the first silicon oxide film;
(C) forming a sidewall made of the second silicon oxide film on the side surface of the first silicon oxide film by performing dry etching on the entire surface of the second silicon oxide film;
And a step (d) of forming a second insulating film so as to cover the first silicon oxide film and the side wall. Forming a silicon oxide film on the first insulating film (a);
A step (b) of forming a mask pattern covering a predetermined region in the silicon oxide film;
A step (c) of selectively removing the upper portion of the silicon oxide film by wet etching using the mask pattern after the step (b);
After the step (c), a step (d) of patterning the silicon oxide film by dry etching using the mask pattern;
And (e) forming a second insulating film so as to cover the patterned silicon oxide film after the step (d). In the formation method of the electret structure of Claim 6 or 7,
The method of forming an electret structure, wherein the first insulating film and the second insulating film are silicon nitride films. A step (a) of patterning a silicon oxide film on the first insulating film;
A step (b) of forming a second insulating film so as to cover the silicon oxide film;
A step (c) of forming a third insulating film so as to cover the second insulating film;
And (d) forming a sidewall made of the third insulating film on a side surface of the second insulating film by performing dry etching on the entire surface of the third insulating film. A method of forming an electret structure characterized by the above. The method of forming an electret structure according to claim 9,
The method of forming an electret structure, wherein the first insulating film, the second insulating film, and the third insulating film are silicon nitride films.

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